The present invention relates to investment casting of metals and alloys using a ceramic investment mold and a melt reservoir connected to the mold by an inverted melt feed gate to provide for bottom feeding of the melt from the reservoir.
In the manufacture of components, such as nickel base superalloy turbine blades and vanes, for gas turbine engines, investment casting techniques have been employed in the past to produce equiaxed, single crystal or columnar grain castings having improved mechanical properties at high temperatures encountered in the turbine section of the engine.
In the manufacture of turbine blades and vanes for modern, high thrust gas turbine engines, there has been a continuing demand by gas turbine manufactures for internally cooled blades and vanes having complex, internal cooling passages including such features as pedestals, turbulators, and turning vanes in the passages in a manner to provide desired cooling of the blade or vane. These small cast internal surface features typically are formed by including a complex ceramic core in the mold cavity in which the melt is cast. The presence of the complex core having small dimensioned surface features to form pedestals, turbulators, and turning vanes or other internal surface features renders filling of the mold cavity about the core with melt more difficult and more prone to inconsistency. Wettable ceramics and increased metallostatic head on the mold and higher preheat temperatures have been used in an attempt to improve mold filling and reduce localized voids in such situations, but these are costly and may be restricted by physical size of the casting apparatus. Moreover, to reduce casting weight, gas turbine engine manufacturers require thinner airfoil wall thickness and smaller cast to size external features that are not possible or very difficult to fill with molten metal.
U.S. Pat. No. 5,592,984 describes a method of investment casting gas turbine engine blades and vanes and other components wherein a ceramic investment mold is disposed in a casting furnace in a casting chamber and filled with the melt with the casting chamber being gas pressurized rapidly enough after casting to reduce localized void regions present in the melt as a result of surface tension effects between the melt and mold components such as ceramic mold and/or core.
Moreover, there is a continuing desire to improve the cleanliness of the melt supplied to the mold cavities in particular to reduce oxide and other inclusion-forming particles in the melt that constitute harmful inclusions in the casting that adversely affect its mechanical properties.
It is an object of the present invention to provide method and apparatus for investment casting using an investment mold and a melt reservoir communicated to the mold by an inverted melt feed gate to provide for cleaner bottom feeding of the melt to the mold and better filling of the mold.
The present invention provides method as well as apparatus for investment casting wherein a ceramic investment mold is disposed in a chamber, and a mold melt reservoir is communicated to one or more mold cavities and includes a reservoir volume for holding at least enough melt, preferably an excess of melt, to fill the mold cavities. The melt reservoir is communicated to the mold cavities via an inverted loop feed passage or gate so that the melt is fed from a lower region of the reservoir through the inverted mold loop feed gate to the mold when the reservoir of melt is gas pressurized. However, the mold loop feed gate is configured to have a loop passage region above the maximum melt level in the reservoir so as to prevent melt flow from the reservoir to the mold cavities in the absence of gas pressurization of the melt. While residing in the melt reservoir, oxides and other inclusion-forming particles in the melt can float to the upper surface of the melt, whereby the melt bottom fed from the lower region of the reservoir to the mold via the inverted loop melt feed gate has a reduced amount of inclusion-forming particles therein. An optional molten metal filter can be used to remove or reduce inclusions in the molten metal fed to the mold without a detrimental loss of molten metal flow since the melt is fed under gas pressurization.
When melt is present in the reservoir, either by being melted in-situ in the reservoir or introduced therein from a crucible, one embodiment of the invention gas pressurizes the chamber in a manner to provide gas pressure on the melt in the reservoir to force the cleaner bottom melt through the inverted mold loop feed passage or gate into the mold cavities to fill same, leaving some dirty melt (melt contaminated with inclusion-forming particles) proximate the upper melt surface remaining in the reservoir. The chamber can be first evacuated during melting of a charge of metallic material in-situ in the reservoir and then gas pressurized by introducing a gas, such as an inert or non-reactive gas, into the chamber at a suitable gas pressure to force melt from the reservoir through the mold loop into the mold cavities. In this embodiment, the mold can be made or treated to have reduced gas permeability such that gas pressurization of the chamber will cause the melt in the reservoir to flow through the loop feed gate into the mold cavities without the need for a pressure cap. An outer refractory glaze or other coating that reduces gas permeability through the mold can be provided on the mold exterior to this end.
The present invention aids in filling of fine details in the mold cavity that are defined by internal mold surface features and/or core surface features that are otherwise difficult to fill with the melt. The present invention also aids in filling the mold with melt having reduced amounts of inclusion-forming particles to provide cleaner castings.